1. Field of the Invention
The present invention relates to the making, in an integrated circuit, of a MOS transistor with high threshold voltage.
A large number of logic MOS transistors are formed in a MOS type integrated circuit. These logic MOS transistors commonly have threshold voltages in the range of some volts. However, in a circuit such as this, it may be useful to have MOS transistors with appreciably higher threshold voltages, of the order of some tens of volts, for example to act as input protection diodes.
Transistors such as these are also necessary to make output transistors for a circuit that has to permanently undergo operating voltages, at its drain and gate, that are greater than the supply voltages of the circuit.
2. Description of the Prior Art
To make MOS transistors with high threshold voltages, structures such as the one shown schematically in FIG. 1 have been used in the prior art. This figure shows a MOS transistor including, in a P- type semiconductor substrate, a source region 2 and a drain region 3 of the N+ type. The gate 4 is formed on a thick oxide or field oxide zone 5, corresponding to the oxide zones normally designed to separate the MOS transistors of an integrated circuit from one another. A source metallization 6 and a drain metallization 7 are also shown. P type channel barrier regions 8 are formed beneath the thick oxide regions external to the high threshold voltage transistor, as well as an overdoped region 9, corresponding to these channel barrier regions, beneath the thick oxide of the gate 4.
To form a protection device, this MOS transistor is connected as a diode, i.e. the drain 3 and the gate 4 are interconnected, as are the source and the substrate.
In principle, a transistor having the structure shown in FIG. 1 exhibits a high threshold voltage owing to its high gate thickness and the enhancement of the channel region. In certain technologies, this threshold voltage may be of the order of 30 to 50 volts. Nevertheless, in practice, this diode-connected transistor will exhibit a far smaller threshold voltage for it is the N+P-(or P-N+) junctions between the drain or the source and the substrate that will break down first, for example at a voltage of the order of 10 to 15 volts, if the P-substrate has a doping concentration of the order of 10.sup.15 atoms/cm.sup.3 and the drain and source regions have a doping concentration of the order of 10.sup.20 at./cm.sup.3.
Thus, if it is desired to manufacture a MOS transistor with high threshold voltage in association with an integrated circuit comprising complementary transistors, there are two possible approaches: either to make zones with a particular doping concentration, distinct from the zones used elsewhere in the integrated circuit, which entails technological steps in addition to the standard steps of manufacture of the integrated circuit, or else to make a device external to the integrated circuit. Both these approaches are costly.